Ascertaining the Position of a Patient in a Magnetic Resonance Apparatus

This application claims the benefit of DE 10 2024 205 802.5 filed on Jun. 21, 2024, which is hereby incorporated by reference in its entirety.

Embodiments relate to a method for ascertaining an item of position information about a position of a patient positioned in a magnetic resonance apparatus, to a magnetic resonance apparatus and to a computer program product.

In medical engineering, imaging by magnetic resonance (MR), also called magnetic resonance imaging (MRI), is characterized by high soft tissue contrasts. A magnetic resonance examination, for example a magnetic resonance measurement, is carried out by a magnetic resonance apparatus, that includes a magnet unit. The magnet unit y includes a main magnet for generating a main magnetic field and a gradient coil unit for generating a gradient magnetic field in an examination region of the magnetic resonance apparatus. The examination region is customarily situated in a magnet bore of the magnet unit, in which a patient is positioned during the magnetic resonance measurement. For generating imaging and/or spectroscopic magnetic resonance signals during a magnetic resonance measurement, radio-frequency (RF) transmit pulses are irradiated by one or more transmit antenna(s) in accordance with a measurement protocol, for example a magnetic resonance sequence, into the examination region, for example into the patient. The RF transmit pulses, that are generated for generating the imaging and/or spectroscopic magnetic resonance signals, customarily have an examination frequency band.

The irradiation of the RF transmit pulses generates RF fields with which nuclear spins in the patient are deflected from their rest position. A subsequent relaxation generates imaging and/or spectroscopic magnetic resonance signals that are received by one or more receive antenna(s) of the magnetic resonance apparatus and are used for reconstruction of magnetic resonance mappings or for spectroscopy.

The generated RF fields cause heating of the tissue of the patient, which is described by a specific absorption rate (SAR). Local intensity maxima may occur primarily in the immediate vicinity of the transmit antennas, which maxima may in the worst case cause burns to the patient. The transmit antennas may be part of a body coil permanently integrated in the magnetic resonance apparatus and that is situated immediately behind an inner wall of a magnet bore of the magnetic resonance apparatus. The transmit power is therefore limited by what is known as contact protection.

Since, according to the prior art, there is no information about the current position of the patient and their actual distances from the inner wall of the magnet bore, often very conservative safety measures are used.

The scope of the embodiments is defined solely by the appended claims and is not affected to any degree by the statements within this summary. The present embodiments may obviate one or more of the drawbacks or limitations in the related art. Independent of the grammatical term usage, individuals with male, female or other gender identities are included within the term.

Embodiments ascertain the position of a patient in order to make, for example, improved SAR monitoring possible.

Accordingly, a method for ascertaining an item of position information about a position of a patient positioned in a main magnetic field of a magnetic resonance apparatus is provided. The magnetic resonance apparatus includes at least one transmit antenna, wherein each of the at least one transmit antennas is provided, for example configured, to generate RF transmit pulses for generating magnetic resonance signals in the body of the patient. Furthermore, the magnetic resonance apparatus includes at least one receive antenna that is configured to receive a response signal of the RF transmit pulse, for example a magnetic resonance signal triggered by the RF transmit pulse. The at least one transmit antenna of the magnetic resonance apparatus generates an RF transmit pulse that includes a frequency band that is outside of an examination frequency band of the magnetic resonance apparatus (hereinafter also called position RF transmit pulse). The item of position information is ascertained using the (possible) response signal of the RF transmit pulse.

The item of position information may be ascertained, for example, with an ascertainment unit. The ascertainment unit may, for example, be part of a system control unit of the magnetic resonance apparatus. The ascertainment unit may, for example, include one or more processor(s) and/or one or more memory module(s).

The ascertained position may be provided and/or saved. For example, the ascertained position may be displayed and/or processed further. For example, a control signal may be generated using the item of position information.

Hereinafter the RF transmit pulse will also be called a position RF transmit pulse in order to clearly differentiate it from any further RF transmit pulses that are not used for ascertaining the item of position information, but instead, for example, for generating imaging and/or spectroscopic magnetic resonance signals; such further RF transmit pulses customarily have the examination frequency band. such further RF transmit pulses are may also be generated with the same at least one transmit antenna with which the position RF transmit pulses are also generated.

The response signal may be a magnetic resonance signal (potentially) triggered by the position RF transmit pulse, for example a signal of a free induction decay (FID) signal. The response signal itself may not be suitable and/or provided for reconstructing magnetic resonance imaging therefrom and/or deriving an item of spectroscopic information therefrom. The response signal itself may not include any medical-diagnostic information.

The item of position information may include, for example, whether a response signal was received or not and/or derived therefrom. The item of position information may include, for example, a property of the response signal, such as the strength of the response signal, and/or be derived therefrom.

The magnetic resonance apparatus may include, for example, a tunnel-shaped and/or a cylindrical, magnet bore in which at least one part of the patient is positioned. The region of the magnet bore may be a patient-receiving region in which the at least one part of the patient is positioned. For example, the part of the patient from which imaging and/or spectroscopic magnetic resonance signals are to be captured with the magnetic resonance apparatus is positioned in the magnet bore. For example, at least one part of the patient is positioned in an isocenter of the magnetic resonance apparatus, for example of the main magnetic field. In the isocenter the main magnetic field, for example, the strength of the main magnetic field, may have a particularly high homogeneity. For example, in the isocenter the main magnetic field includes a homogeneity of less than 2 ppm.

The magnet bore may be delimited by an inner wall. The inner wall may be situated between a patient-receiving region in which at least one part of the patient is positioned, and the at least one receive antenna. For example, the at least one receive antenna is a constituent part of a body coil that is permanently integrated in a magnet unit of the magnetic resonance apparatus. The magnet unit may surround the patient-receiving region and/or delimit the magnet bore.

The examination frequency band of the magnetic resonance apparatus includes the frequencies with which, for generating imaging and/or spectroscopic magnetic resonance signals, nuclear spins precess around the direction of an outer magnetic field (hereinafter also called Larmor frequency, resonance frequency and/or precession frequency). Frequencies outside of the examination frequency band of the magnetic resonance apparatus may also be referred to as off-resonant. Outside of the examination frequency band are frequency ranges that are outside of the frequency range customarily used for imaging and/or spectroscopy.

The Larmor frequency is dependent on the nucleus type and the strength of the applied magnetic field, for example of the main magnetic field. At 1.0 tesla the Larmor frequency of protons is, for example, approx. 42 MHz, at 1.5 tesla approx. 63 MHz. The Larmor frequency may be calculated with equation (1): f=γ/(2π)B, where γ is the gyromagnetic ratio and B the strength of the magnetic field.

An appropriate Larmor frequency may therefore be associated with a magnetic field strength. With a homogeneous magnetic field strength, the associated Larmor frequency is therefore also homogeneous. Consequently the associated Larmor frequency also includes a high homogeneity in the isocenter.

Customarily, there is a localized dependence of the strength of the main magnetic field. As already explained, in the region of the isocenter the strength of the main magnetic field is customarily particularly homogeneous. Customarily, the strength of the main magnetic field is increasingly more inhomogeneous outwardly, for example in the direction of the inner wall of the magnet bore. There is thus a correlation between location and strength of the magnetic field, for example of the main magnetic field.

Advantageously, this correlation, i.e. the spatial distribution of the strength of the main magnetic field, is known. Advantageously, the strength of the main magnetic field is provided for ascertaining the item of position information. Advantageously, the item of position information is ascertained using the spatial distribution of the strength of the main magnetic field.

Advantageously, the selection of the frequency band of the position RF transmit pulse makes it possible to determine in which spatial region nuclear spins, for example protons, are excited by magnetic resonance. The response signal results from such an excitation. For example, only nuclear spins, that are situated in a specific spatial region, for example outside of the isocenter, are excited with the position RF transmit pulse. For example, the response signal is a magnetic resonance signal of protons (for example in the body of the patient) that, in this specific spatial region, are situated, for example, outside of the isocenter. If the patient moves a body part, for example in such a way that it is situated in the specific region, for example outside of the isocenter, nuclear spins may be excited in this body part by the position RF transmit pulse. Advantageously, an item of position information may then be ascertained that includes an item of information that this body part is situated in the specific region, for example outside of the isocenter.

Advantageously, the size of the spatial region in which nuclear spins, for example protons, are excited by magnetic resonance, may also be determined via the width of the frequency band of the position RF transmit pulse. The wider the frequency band of the RF transmit pulse is, the larger the spatial region monitored by the RF transmit pulse also customarily is with a constant magnetic field distribution, since equation (1) is satisfied for a plurality of frequencies and thereby a plurality of locations.

The frequency band of the position RF transmit pulse is outside of the examination frequency band of the magnetic resonance apparatus. A magnetic resonance excitation outside an imaging volume of the magnetic resonance apparatus takes place due to the position RF transmit pulse. By contrast, imaging and/or spectroscopic magnetic resonance signals are generated customarily with RF transmit pulses whose frequency components are in the examination frequency band. A magnetic resonance excitation takes place in the imaging volume of the magnetic resonance apparatus due to RF transmit pulses whose frequency components are in the examination frequency band.

Advantageously, the magnetic resonance apparatus includes a transmit system, for example including the at least one transmit antenna, that is configured to capture the position RF transmit pulse as well as RF transmit pulses for generating imaging and/or spectroscopic magnetic resonance signals. The transmit system includes a bandwidth within which lie the frequency band of the position RF transmit pulse and the examination frequency band of the magnetic resonance apparatus.

Advantageously, the magnetic resonance apparatus includes a receive system, for example including the at least one receive antennas, that is configured to capture the response signal of the position RF transmit pulses as well as imaging and/or spectroscopic magnetic resonance signals. The receive system includes a bandwidth within which lie the frequency band of the position RF transmit pulse and the examination frequency band of the magnetic resonance apparatus.

Advantageously, response signals, for example magnetic resonance signals, are acquired from strongly off-resonant regions and associated with the patient, from which it is possible to infer whether at least a part of the patient is situated in specific local regions or not. Advantageously, no additional hardware with respect to a conventional magnetic resonance apparatus is required for carrying out the method.

Advantageously, the frequency band of the RF transmit pulse includes a distance from the center frequency of the examination frequency band, with the distance being selected such that the response signal includes an item of position information about a spatial region of interest and/or that is to be monitored. For example, the frequency band of the RF transmit pulse is in a region of ±1,000-2,500 ppm relative to the center frequency of the examination frequency band. In this connection “ppm” means “parts per million” and/or “millionth”.

Advantageously, the distance is selected such that a spatial region that is relevant to the safety of the patient is covered thereby. Advantageously, the distance is selected such that a spatial region that constitutes a danger to the patient if the body of the patient is situated wholly or partially therein is covered thereby.

The frequency band of the RF transmit pulse includes a distance from the center frequency of the examination frequency band of the magnetic resonance apparatus of at least 100 ppm, for example at least 200 ppm, for example at least 500 ppm.

The item of position information is ascertained during a magnetic resonance measurement. For example, a measurement protocol, for example a magnetic resonance sequence, may be applied during the magnetic resonance measurement, according to which imaging and/or spectroscopic magnetic resonance signals are captured with the aid of RF transmit pulses in the examination frequency band and response signals are captured with the aid of position RF transmit pulses outside of the examination frequency band.

The imaging and/or spectroscopic magnetic resonance signals as well the response signals may be captured successively. Advantageously, the RF transmit pulses for generating the response signals are switched on between the other RF transmit pulses generated without interfering with the capture of the imaging and/or spectroscopic magnetic resonance signals.

Generation of the position RF transmit pulse and ascertainment of the item of position information may be carried out repeatedly during the magnetic resonance measurement of the patient. The more often response signals are generated and received, the more continuously the position of the patient may be monitored.

The frequency band of the RF transmit pulses may be varied with repeated generation of the position RF transmit pulse and ascertainment of the item of position information. Advantageously, it is thus possible to monitor different spatial regions. Thus, for example with a first position RF transmit pulse, that includes a first frequency band, it is possible to monitor a first region, and with a second position RF transmit pulse, that includes a second frequency band (different from the first frequency band), it is possible to monitor a second region (different from the first region).

Advantageously, a variation in the frequency band of the RF transmit pulse may improve the spatial resolution of the monitoring. For example, the frequency bands may be reduced, so the associated regions also become smaller. Conversely, it is possible to emit more RF transmit pulses in order to obtain the overall coverage of the monitoring.

The magnetic resonance apparatus may include a magnet bore within which at least one part of the patient is positioned, wherein the item of position information includes an item of distance information about the distance of the patient, for example any body part of the patient, from an inner wall of the magnet bore.

For example, the distance of the frequency band of the position RF transmit pulse from the center frequency of the examination frequency band is selected such that a possible response signal is generated in a region close to the inner wall of the magnet bore. If the patient moves, for example, a body part, for example an arm, into this region close to the inner wall of the magnet bore, the position RF transmit pulse may generate a response signal, for example a magnetic resonance signal, by interacting with nuclei, for example protons, in the body part. The response signal may be received by the magnetic resonance apparatus, and the item of position information may be ascertained using the response signal. Even if no response signal is received, an item of position information exists since it is possible to infer from the missing response signal that no tissue generating a response signal is situated in the spatial region of interest and/or that is to be monitored.

In this way it is possible to achieve contact protection of the inner wall of the magnet bore, for example delimitation of the transmit power. Instead of adopting a conservative safety assumption for the SAR monitoring, the actual position of the patient, for example their body parts, may be taken into account. Advantageously, more effective magnetic resonance protocols may be applied thereby.

One possible embodiment of the method provides that the magnetic resonance apparatus includes at least one gradient coil, wherein a magnetic field gradient is generated in the main magnetic field during the generation of the RF transmit pulse with the at least one gradient coil.

Advantageously, a slice-selective excitation of response signals by the position RF transmit pulse may be carried out by the generation of the magnetic field gradients. The item of position information may consequently be improved, for example the position of the patient may be ascertained more accurately.

For example, the magnet bore includes a center axis and/or a longitudinal axis (z-axis). This may run parallel to the surface of the magnet bore. (In the case of a circular cylindrical magnet bore, the center axis and/or longitudinal axis would run through the center of the circular base area of the cylinder.) The generated magnetic field gradient runs along the center axis and/or a longitudinal axis of the magnet bore. Advantageously, a slice may be selected in the z-direction with the aid of such a magnetic field gradient.

Advantageously, the item of position information may be improved by such a slice selection. For example, any excitation regions close to the inner wall of the magnet bore may thus be separated from those that have a greater distance from the inner wall of the magnet bore, but a different position along the z-axis.

One possible embodiment of the method provides that the at least one receive antenna includes a plurality of receive antennas whose positions relative to the magnetic resonance apparatus, for example to the magnet bore, are known. Each of the plurality of receive antennas is configured to receive a partial response signal of the response signal, with the item of position information being ascertained using the (possible) partial response signals and the positions of the receive antennas. Advantageously, the ascertained item of position information may consequently be improved.

The partial response signals may be received by the plurality of receive antennas. For example, each of the plurality of receive antennas may be associated with one receive channel. The magnetic resonance apparatus may include, for example, 8, 16 or 32 receive channels with one receive antenna respectively. The item of position information may be ascertained with the aid of a coil sensitivity profile of the plurality of receive antennas.

Advantageously, an item of information about the point of origin of the response signal may be derived from the known position of the respective receive antenna and the partial response signal received by the receive antenna. Advantageously, the partial response signals may be correlated with each other and/or compared to each other such that their point of origin may be inferred. All partial response signals of the response signal may have the same root cause, for example the same relaxation process of one or more nuclear spin(s) excited by a position RF pulse (for example of nuclei in the body of the patient, if it is situated in a corresponding region that is affected by magnetic resonance and generation of a response signal resulting therefrom).

If, for example, a first partial response signal, that was received by a first receive antenna, includes a larger amplitude than a second partial response signal, that was received by a second receive antenna, then it is possible to infer from this, for instance, that the point of origin of the response signal including the two partial response signals is located closer to the position of the first receive antenna.

The plurality of receive antennas may be constituent parts of one or more local coil(s). Local coils may be provided to be arranged directly on the body of the patient. The partial response signals are received by receive antennas in one or more local coil(s). The one or more local coil(s), for example the receive antennas, may be arranged directly on the body of the patient.

Furthermore, a magnetic resonance apparatus is provided that is configured to carry out a method as described above. The magnetic resonance apparatus may include, for example, at least one transmit antenna for generating RF transmit pulses and/or at least one receive antenna for receiving magnetic resonance signals. Furthermore, the magnetic resonance apparatus may include, for example, a magnet bore, inside of which at least one part of the patient may be positioned. Furthermore, the magnetic resonance apparatus may include, for example, a system control unit for controlling generation of RF transmit pulses, for evaluating received response signals of the RF transmit pulses, for example for ascertaining an item of position information using the response signals.

The advantages of the proposed magnetic resonance apparatus substantially correspond to the advantages of the method described above for ascertaining an item of position information about a position of a patient positioned in a main magnetic field of a magnetic resonance apparatus, that are stated above in detail. Features, advantages or alternative embodiments mentioned in this connection may likewise also be transferred to the other claimed subject matter, and vice versa.

Furthermore, a computer program product is provided that includes a program and may be loaded directly into a memory of a programmable system control unit of a magnetic resonance apparatus and includes program code, for example libraries and auxiliary functions, in order to carry out a proposed method when the computer program product is executed in the system control unit of the magnetic resonance apparatus. The computer program product may include software with a source code that still has to be compiled and linked or that only has to be interpreted, or executable software code that only has to be loaded into the system control unit for execution.